One of the most significant developments in the field of archery has been the development of precision arrow shafts in which the degree of flex can be held to a close tolerance known as “spline.” With the advent of such precision arrows, it is now possible to match a given bow to a particular precision arrow to optimize accuracy and performance. This is particularly important in modern compound bows in which the amount of force applied by the bowstring varies with the position of the bowstring with respect to the bow.
In order to correctly select an arrow for a specific bow, it is necessary to determine the amount and rate of energy transfer from the bowstring to the arrow. This has been done using charts developed for the purpose, such as the Easton® Shaft Selection Charts. Using these charts, the archer chooses from among a coarse selection of bow cams which the archer estimates most closely corresponds to his or her bow. The archer then goes on to answer specific questions about his bow, and the respective chart will ultimately lead him to a general arrow selection. These chart-based arrow selection systems have also been automated in the form of computer programs such as the Easton® Shaft Selector system and the Archer's Advantage™ program. In all cases, an attempt is made to estimate a force-draw curve of the bow in order to select an optimum arrow. The force-draw curve comprises a plot of the force applied by the bowstring as the bowstring is drawn from an initial relaxed position to the maximum bowstring displacement. More particularly, the force-draw curve is defined by the Archery Manufacturers and Merchants Organization to be the curve that is plotted using the force readings, taken at incremental values of draw length when drawing the bow, as the ordinate and the corresponding draw length as abscissa.
Ideally, one would prefer to actually measure the force-draw curve of the bow, as opposed to estimating it. Knowing the force-draw curve of the bow allows a more refined arrow selection than is possible with a chart-based estimating system. Standards for determining the force-draw curve of a bow are set in the document entitled “Standard Test Method for Determining the Force-Draw and Let-Down Curves for Archery Bows” Designation: F 1832-97 (reapproved 2002) published by ASTM International. This publication also sets a standard for determining the “let-down curve” of a bow. The let-down curve is defined as the curve that is plotted using the force readings, taken at incremental values of draw length when relaxing the bow, as the ordinate and the corresponding draw length as abscissa. The entire content of this publication is hereby incorporated herein by this reference.
U.S. Pat. No. 6,220,235 shows a device that may be used to create the actual force-draw curve for a bow. In the device shown in this patent, the bow is attached to a fixture and a force measuring device is attached between the bowstring and a pulley system by which the bowstring may be drawn. The fixture also includes a scale that may be used to measure the displacement of the bowstring in the course of drawing the bowstring to a maximum displacement using the pulley system, and then returning the bowstring to the relaxed position. By combining the force reading from the force measuring device with the bowstring position data, both the actual draw-force curve and the actual let-down curve for the bow may be plotted.
Although it is possible to plot the actual draw-force curve and let-down curve for a bow using the device shown in U.S. Pat. No. 6,220,235, the device required to collect the data is cumbersome and mechanically complex. Many archers do not have access to such a fixture-based device and thus opt for the less accurate chart-based arrow selection in lieu of selection based on the actual force characteristics of their bow and bowstring.